1. Field of the Invention
The present invention relates to a protection circuit, more particularly to an overcurrent protection circuit of a battery.
2. Description of Related Art
It is well known that a Lithium ion battery has been broadly used as a battery cell of a battery pack, since the Lithium ion battery generates a high energy density per weight and volume, and generally provides a reduction in size and weight of a portable type apparatus. However, the Lithium ion battery has a safety problem that its performance characteristic is degraded when it is charged by an overvoltage, and what is more, it even tends to become explosive if it is operated beyond its capacity. Hence, every Lithium ion battery cell needs a protection circuit. The protection circuit commonly comprises charging overvoltage protection, discharging overvoltage protection, charging overcurrent protection, discharging overcurrent protection and short circuit protection.
The discharging overcurrent protection is that when the battery is discharged through a load resistor, the discharging current exceeds a related overcurrent voltage protection threshold VEDI and this state keeps a period of time beyond a certain delay time TEDI, the battery protection circuit switches off the discharging path to prohibit discharging, thereby entering the overcurrent protection state. When the discharging current is further increased beyond a related short voltage protection threshold VSC and this state keeps a period of time beyond a certain delay time TSC, the battery protection circuit switches off the discharging path to prohibit charging, thereby entering the short protection state. The short protection state and the overcurrent protection state are the same for a control circuit and both aim to prohibit discharging in the circuit. Exiting conditions of the two protection state are the same too and both are that the voltage drop between the VM node and the G node is less than the voltage threshold VEDI and this state keeps a period of time beyond a certain delay time. The main difference of the two states is that the short voltage protection threshold VSC is larger than the overcurrent voltage protection threshold VEDI, and the delay time TSC is less than the delay time TEDI. Namely, the more the discharging overcurrent is, the shorter the delay time is.
In the prior art, several discharging overcurrent thresholds and one short threshold may be configured for discharging overcurrent protection. However, other overcurrent states do exist among the several protections. The conventional protection scheme can not efficiently protect the battery from discharging overcurrent especially in the intermediate states.
Thus, improved techniques for overcurrent protection circuit having continuous protection thresholds are desired to overcome the above disadvantages.